Cold hardiness & minimum temperature

Is Borage (Borago officinalis)cold hardy? Hardiness zone & min temp

Also called Starflower, Bee Bush.

More about borage

About Borage

Borago officinalis · also called Starflower, Bee Bush · herb

Borage is a fast, bushy hardy annual grown for its bristly cucumber-flavoured leaves and edible, star-shaped blue flowers that pollinators adore. It germinates and blooms quickly in full sun and poor soil, self-seeds prolifically, and resents transplanting. Note: despite culinary use, it is ASPCA-listed as toxic to pets, so keep it away from grazing cats and dogs.

Cold limit: USDA 2-11 (grown as a warm-season annual) · RHS H4 (15-27°C)

What borage's hardiness rating actually means

Yes — borage is genuinely cold hardy. Rated RHS H4 and USDA 2-11 (grown as a warm-season annual), it lives outdoors all year and needs winter cold rather than protection from it. Its RHS rating of H4 means: Hardy in an average winter across much of the temperate world. On the US scale that maps to USDA 2-11 (grown as a warm-season annual) — the zones where it can be left outdoors year-round.

New to these scales? The USDA hardiness zone map explained covers how the zone numbers work, and you can find your own zone with the zone finder.

Minimum temperature — and what happens below it

Minimum survivable temperature is roughly about −10 to −5 °C. Borage is built for winter — once established it takes hard frost and snow in its stride.

Concretely, for borage as it gets too cold:

It tolerates winter lows to about −10 to −5 °C once established.
Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root.
First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.

Can borage go outside or overwinter — and where?

Plant it out within USDA 2-11 (grown as a warm-season annual) and it overwinters with little or no help.
It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy.
The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

Work back from your local frost dates with the frost-date calculator: the last spring frost and first autumn frost are what really decide when borage can be outside. US growers can check USDA zones; UK growers should use the RHS hardiness ratings, which match the H4 figure above.

Borage hardiness — frequently asked questions

Is borage cold hardy?

Yes — borage is genuinely cold hardy. Rated RHS H4 and USDA 2-11 (grown as a warm-season annual), it lives outdoors all year and needs winter cold rather than protection from it. An outdoor plant. Borage is hardy across USDA 2-11 (grown as a warm-season annual); it belongs in the ground or a frost-proof container, not on a windowsill, and many types actively need a cold winter to perform.

What is the minimum temperature borage can survive?

Minimum survivable temperature is roughly about −10 to −5 °C. Borage is built for winter — once established it takes hard frost and snow in its stride.

What hardiness zone is borage?

Borage is rated USDA 2-11 (grown as a warm-season annual) and RHS H4 — Hardy in an average winter across much of the temperate world.

Can borage survive winter outside?

Plant it out within USDA 2-11 (grown as a warm-season annual) and it overwinters with little or no help. It does not want to come indoors — a warm winter room actually weakens a hardy plant by denying it dormancy. The real risks in its range are waterlogging, wind-rock on young plants, and a late hard frost on new growth — not ordinary winter cold.

What happens to borage below its minimum temperature?

It tolerates winter lows to about −10 to −5 °C once established. Below its rated zone, the visible damage is browned or blackened top growth and, in the worst case, a killed crown or root. First-year, newly planted, or container-grown specimens are noticeably less hardy than established garden plants — the roots are exposed.